This invention is directed to ultrasonic techniques for detecting flaws on a surface, and in particular, to a method and laser apparatus for generating surface acoustic waves.
Conventional ultrasonic and eddy current techniques have been used for detecting flaws in various types of objects. However, these techniques require contact with the object and, therefore, cannot be used when non-contact is desirable or necessary, as with objects at high temperatures.
Non-contact techniques have been developed in which bulk acoustic waves are generated in a material by a laser and the acoustic waves reflected by the inner flaws are detected by interferometry. The generation of bulk acoustic waves is described by interferometry. The generation of bulk acoustic waves is described by W. Kaule et al in U.S. Pat. No. 4,121,469 which issued on Oct. 4, 1978; by R. L. Melcher et al in U.S. Pat. No. 4,137,991 which issued on Feb. 6, 1979; and by W. Kaule et al in U.S. Pat. No. 4,169,662 which issued on Oct. 2, 1979. The interferometric method of detecting acoustic waves in a material is described by C. M. Penney in U.S. Pat. No. 3,978,713 which issued on Sept. 7, 1976; and by E. Primback in U.S. Pat. No. 4,180,324, which issued on Dec. 25, 1979. This non-contact technique has the advantages of better repetitivity of the measurement because of the absence of a coupling liquid, ease of scanning, access to concave or irregular surfaces, and large and flat frequency response leading to an improved spatial and temporal resolution.
It has been found, however, that the detection of cracks or flaws in surfaces may best be carried out by using surface or Rayleigh acoustic waves, as described in the publication by B. R. Tittman et al, "Fatigue Lifetime Prediction with the Aid of Surface Acoustic Wave NDE", Journal N.D.E., 1, 123 (1980). Surface acoustic waves are difficult to generate by conventional piezoelectric methods, especially at high frequencies, because of precise angular alignment and need for a liquid couplant which strongly attenuates surface waves. The laser generationof surface acoustic waves has not been very successful to date as outlined in the publication by A. M. Aindow et al "Laser-Generated Ultrasonic Pulses at Free Metal Surfaces", J. Acoust. Soc. Am., 69, 449, 1981, because of low efficiency, low frequency and difficulty to discriminate against bulk waves.